1. Field of the Invention
This invention relates to separation of a nitrated substance from a spent nitrating acid in which the nitrated substance is soluble. More particularly, this invention relates to such separation wherein the nitrated substance contains strongly polar substituents which would be expected to cause the nitrated substance to remain in the spent acid when attempts are made to extract it with a solvent.
2. Discussion of the Prior Art
In the preparation of amines, particularly amine dyes and pigments, it is known in the prior art nitrate a desired substance with nitric acid. Since water is formed during nitration and also retards reaction, it is desirable to tie this water up during nitration. Accordingly, the nitration is generally carried out in the presence of an additional strong acid such as sulfuric acid or phosphoric acid. After nitration the nitric acid is, in general, depleted and the remaining acid which is primarily additional strong acid is referred to as spent nitrating acid. The nitrated substance is then separated from the spent acid, followed by reduction to the amine. One conventional reduction method is iron-acid reduction and another is by catalytic hydrogenation.
In accordance with prior art practice after nitration the reaction mixture generally contains by weight about 5 to 80 percent nitrated substance, about 10 to 70 percent spent nitrating acid and about 5 to 15 percent water. This mixture is diluted with about 3 times its weight of additional water and neutralized with limestone and soda ash. The resulting precipitate is removed by filtration and the filtrate is further processed generally by the iron-acid reduction process to produce amines from the nitrated substance. This process has the disadvantages that:
(a) substantial loss of product occurs due to retention of nitrated substance in the gypsum cake; PA1 (b) the gypsum disposal presents environmental problems and PA1 (c) the strong acid and limestone are irreversibly consumed. PA1 (1) diluting the nitrated substance-spent acid solution with water, PA1 (2) adding a monohydric alcohol or ketone extraction agent which is immiscible with the spent acid, PA1 (3) permitting the mixture to separate into layers of different densities one of said layers comprising said nitrated substance dissolved in said extracting agent, PA1 (4) removing said nitrated substance-extracting agent layer, PA1 (5) separating the nitrated substance from the extracting agent layer, and PA1 (6) reducing the nitrated substance to an amine.
Accordingly it would appear that extraction with a solvent would be an alternative which would avoid these disadvantages. However, some of the nitrated substances of greatest interest contain polar substituents such as the sulfonic acid group, as in 2-chloro-5-nitrotoluene-4-sulfonic acid, --OH, --SH, --CO.sub.2 H, --OSO.sub.3 H, and --OP(OH).sub.2 which render the nitrated compound very soluble in aqueous systems and relatively insoluble in most of the solvents which are immiscible with water.
Accordingly, it is the purpose of the instant invention to provide a process for removing a nitrated substance from a solution of a strong acid in which it is soluble when said nitrated substance contains polar substituent groups which would normally be expected to cause the nitrated substance to remain in the acid layer.